Marine seismic surveys are usually conducted by towing an energy source and seismic detectors behind a vessel. The source imparts an acoustic wave to the water, creating a wavefield which travels coherently into the underlying earth. As the wavefield strikes interfaces between earth formations, or strata, it is reflected back through the earth and water to the detectors, where it is converted to electrical signals and recorded. Through analysis of these signals, it is possible to determine the shape, position and lithology of the sub-bottom formations. In other marine survey methods, the detectors and/or sources are placed at or close to the sea bottom or in wells.
A problem encountered in marine surveying--as well as in inverse vertical seismic profiling or "VSP"--is that of water column reverberation. The problem, which arises as a result of the inherent reflectivity of the water surface and bottom, may be explained as follows. A seismic wave generated in (or reflected of) earth strata passes into the water in a generally upward direction. This wave, termed the "primary", travels through the water and past the seismic detector which records its presence. The wavefield continues upward to the water's surface, where it is reflected back downwards. This reflected, or "ghost", wavefield also travels through the water and past the detector(s), where it is again recorded. Depending upon the nature of the earth material at the water's bottom, the ghost wavefield may itself be reflected upwards through the water, giving rise to a series of one or more subsequent ghost reflections or multiples.
Free-surface multiple reflections can be classified according to their order, which is equal to the number of reflections from the free-surface. Thus, first order free-surface reflections comprise energy initially travelling downwardly from the sources (as opposed to "ghosting" where energy travels upwardly and is reflected from the free surface), is reflected upwardly from the sea bed or a boundary below the sea bed, and is then reflected downwardly from the free-surface to the hydrophones. Second order free-surface multiple reflections undergo two downward reflections from the sea-surface before being detected by the hydrophones, and so on.
This reverberation of the seismic wavefield in the water obscures seismic data, amplifying certain frequencies and attenuating others, thereby making it difficult to analyse the underlying earth formations.
In instances where the earth material at the water bottom is particularly hard, excess acoustic energy or noise generated by the seismic source can also become trapped in the water column, reverberating in the same manner as the reflected seismic waves themselves. This noise is often high in amplitude and, as a result, rends to cover the weaker seismic reflection signals sought for study.
In the art, Ruehle, in U.S. Pat. No. 4,486,865, discloses a technique for reducing ghosting wherein a pressure detector and a particle velocity detector positioned in close proximity to one another in the water. The output of at least one of the detectors is gain-adjusted and filtered, using a deconvolution operation having a predetermined amount of white noise to the zero lag of the autocorrelation function. The patent suggests that, by adding this deconvolved/gain-adjusted signal to the output of the other detector, ghost reflections may be cancelled. Dragoset, in U.S. Pat. No. 5,365,492, describes an iterative method of determining an scaling factor used to sum simultaneous pressure and velocity measurements.
Haggerty, in U.S. Pat. No. 2,757,356, discloses a marine seismic reflection surveying system in which two seismometer spreads are disposed at two distinct depths in the water such that water column reverberations received by them are 180 degrees out of phase. By combining the output of the detectors, the patent suggests that the reverberations will cancel.
White, in `Seismic Wave Radiation--Transmission and Attenuation`, McGraw-Hill, 1965, and later Barr, in U.S. Pat. No. 4,979,150, both propose the use of hydrophone/geophone pair to separate upwardly and downwardly travelling waves. Both use a plane wave decomposition of the wave equation to determine a scaling factor which allows to add measurements from the different detectors types. The suggested solution assumes however normal incidence of the waves.
Ikelle et al., in the UK Patent Application GB-A-9710435, use a Born series approach to eliminate multiples from sea-bottom surveys.
Berni, in U.S. Pat. No. 4,345,473, suggests the use of a vertical component accelerometer in combination with hydrophone for cancelling surface-reflected noise in marine seismic operations.
Gal'perin, in "Vertical Seismic Profiling," Special Publication No. 12 of the Society of Exploration Geophysicists, suggests the use of a seismic detector which combines the output of a pressure and velocity sensor for use in VSP surveying.
Moldoveanu, in U.S. Pat. No. 5,621,700, uses a pressure and velocity sensitive sea-bottom cable. The reverberations are attenuated by an averaging process involving the steps of adding the product of the pressure data times the absolute value of the velocity data and the product of the velocity data times the absolute value of the pressure data.
L. Amundsen and A. Reitan published in Geophysics, Vol. 60, 2, 1995, 563-572 a method for decomposing multi-component sea floor data, employing decomposition filters determined by plane wave analysis.
K. M. Schalkwijk et al. published in SEG, Expanded Abstracts, 1997, p. 8-11, a method of simultaneously decomposing the recorded wavefield into up- and downgoing and compressional (P-) and shear (S-) waves in a one-step decomposition. The described method did not provide satisfactory results when applied to real data.
In view of the above-cited references, it is seen as an object of this invention to provide methods of eliminating ghosts or multiples from marine surveys, particularly sea bottom measurements. It is a specific object of the invention to provide such a method without making assumption about the direction of incidence of the acoustic waves.